The determination of the ammonium content has become a very important routine task of environmental analysis, particularly water analysis. The determination can here be carried out by three different methods:
separating off the ammonia by distillation in an alkaline medium, followed by an acidimetric titration PA1 potentiometric determination using ion-selective electrodes or PA1 colorimetric or photometric determination after formation of a coloured compound.
The determinations based on colour formation have gained increasing importance in recent years. The colour reactions used here for ammonium ions are generally the methods of Nessler or Berthelot. Owing to the high detection sensitivity, the high selectivity and the relatively low susceptibility to interference, the Berthelot reaction in particular has become established as detection process for ammonium ions. The fields of application are manifold, e.g. determination of ammonium ions in water, food, soil extracts, biological materials, etc.
The formation of a blue colour on mixing ammonium ions, hypochlorite and phenol was described by Berthelot as early as 1859. The elucidation of the reaction mechanism was thereafter the subject of numerous studies (Anal. Chem. 49, 464 (1977)). A prerequisite for the reaction proceeding is a free para-position on the phenol used. Depending on the hypochloride source used and on the phenol component used, an optimum reaction requires adherence to characteristic reaction conditions such as pH and time sequence of the reagent additions. The phenol component used is generally phenol itself, salicylic acid and thymol. With regard to a high detection sensitivity and high reaction rate, a series of further phenol derivatives have been examined in detail in the prior art for their suitability for quantitative detection of ammonium ions. Among these, 2-methylphenol, 2,6-dimethylphenol, 2-chlorophenol, 2,6-dichlorophenol, guaiacol, o-phenylphenol, m-cresol, 1-naphthol and 2-methyl-5-hydroxyquinoline have, in particular, been found to be usable (Analyst 109, 549 (1984)).
There have been many attempts to optimize the Berthelot reaction by selection of suitable reagents and reaction conditions so as to make possible its use in continuous measurement systems and commercially available test kits. However, its use has hitherto remained the province of wet chemistry, i.e. for reactions in which liquid reactants are exclusively used.
Recently, however, analysis using solid absorptive supports, the so-called test strips, has gained increasing importance. The significant advantages of the use of test strips include, in particular, simple handling and unproblematical disposal owing to the small amounts of reagent. In these strips, all or part of the reagents necessary for the detection reaction are applied to absorptive or swellable supports or films. After contact of the reaction zone with the sample, the detection reaction proceeds. The colour detection reaction proceeds. The colour formed is a measure of the amount of analyte to be determined and can be evaluated visually with the aid of colour comparison scales or using simple reflectometers.
Test strips for detecting ammonium amounts based on the Nessler reaction are known, but they are suitable only for the detection of relatively high ammonium ion contents of more than 20 mg/l. Owing to the usually low ammonium ion contents of water samples, these can therefore frequently not be employed. Multilayer test strip systems exist for the determination of ammonium ions in blood plasma, but the detection in this system is based on the colour change of a pH indicator (bromophenol blue).